In a motor vehicle which is being propelled by a diesel engine, diesel exhaust after-treatment may include a device, such as a diesel particulate filter (DPF), for trapping particulate matter so that the trapped matter doesn't escape into the surrounding atmosphere. Soot is a particulate constituent of engine-out exhaust and may become a significant constituent during certain engine operating conditions.
A DPF is occasionally regenerated in order to maintain soot trapping efficiency. Regeneration may occur naturally under conditions conducive to natural regeneration, but regeneration may also be forced when the quantity of trapped soot reaches a level which begins to degrade engine performance and/or soot trapping efficiency. Regeneration is forced by creating conditions which will burn off trapped soot. If a DPF is regenerated too frequently, fuel may be wasted, but if a DPF is not regenerated frequently enough, it may become clogged to a point where it can't be regenerated, and then has to be removed from the vehicle and baked in a special oven to unclog it.
As a diesel engine operates, an engine control system may from time to time calculate quantity of trapped soot to determine if regeneration should be forced.
A known strategy for determining a quantity of soot trapped in a DPF is based on pressure-flow relationships. For a given exhaust flow rate through a DPF, the difference between DPF inlet pressure and DPF outlet pressure is an indication of a quantity of soot trapped in the DPF. When an engine is operating in a steady state condition, i.e. at a substantially constant speed and a substantially constant load, pressure across, and flow through, a DPF are substantially constant. Sufficiently accurate measurements of those parameters can enable a sufficiently accurate calculation of trapped soot quantity to be made.
However, the manner in which motor vehicles are typically driven results in their engines not always operating in such a steady state condition. While steady state operation occurs during certain driving situations such as highway cruising, vehicle accelerations and decelerations create transients in engine operation. Accuracy of a calculation of trapped soot quantity made during a transient operating condition is problematic.